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3.48

master
Marc Alexander Lehmann 2008-10-30 08:09:30 +00:00
parent 91c5de8bc5
commit 36589b7a20
4 changed files with 365 additions and 164 deletions
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@ -1,5 +1,7 @@
Revision history for libev, a high-performance and full-featured event loop.
WISH? monotonic clocks times/GetTickCount for coarse corrections?
3.48 Thu Oct 30 09:02:37 CET 2008
- further optimise away the EPOLL_CTL_ADD/MOD combo in the epoll
backend by assuming the kernel event mask hasn't changed if

2
configure.ac
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@ -1,7 +1,7 @@
AC_INIT
AC_CONFIG_SRCDIR([ev_epoll.c])
AM_INIT_AUTOMAKE(libev,3.45)
AM_INIT_AUTOMAKE(libev,3.48)
AC_CONFIG_HEADERS([config.h])
AM_MAINTAINER_MODE

480
ev.3
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@ -132,7 +132,7 @@
.\" ========================================================================
.\"
.IX Title "LIBEV 3"
.TH LIBEV 3 "2008-10-21" "libev-3.45" "libev - high performance full featured event loop"
.TH LIBEV 3 "2008-10-30" "libev-3.48" "libev - high performance full featured event loop"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
@ -151,14 +151,14 @@ libev \- a high performance full\-featured event loop written in C
\& #include <ev.h>
\&
\& // every watcher type has its own typedef\*(Aqd struct
\& // with the name ev_<type>
\& // with the name ev_TYPE
\& ev_io stdin_watcher;
\& ev_timer timeout_watcher;
\&
\& // all watcher callbacks have a similar signature
\& // this callback is called when data is readable on stdin
\& static void
\& stdin_cb (EV_P_ struct ev_io *w, int revents)
\& stdin_cb (EV_P_ ev_io *w, int revents)
\& {
\& puts ("stdin ready");
\& // for one\-shot events, one must manually stop the watcher
@ -171,7 +171,7 @@ libev \- a high performance full\-featured event loop written in C
\&
\& // another callback, this time for a time\-out
\& static void
\& timeout_cb (EV_P_ struct ev_timer *w, int revents)
\& timeout_cb (EV_P_ ev_timer *w, int revents)
\& {
\& puts ("timeout");
\& // this causes the innermost ev_loop to stop iterating
@ -182,7 +182,7 @@ libev \- a high performance full\-featured event loop written in C
\& main (void)
\& {
\& // use the default event loop unless you have special needs
\& struct ev_loop *loop = ev_default_loop (0);
\& ev_loop *loop = ev_default_loop (0);
\&
\& // initialise an io watcher, then start it
\& // this one will watch for stdin to become readable
@ -242,7 +242,7 @@ configuration will be described, which supports multiple event loops. For
more info about various configuration options please have a look at
\&\fB\s-1EMBED\s0\fR section in this manual. If libev was configured without support
for multiple event loops, then all functions taking an initial argument of
name \f(CW\*(C`loop\*(C'\fR (which is always of type \f(CW\*(C`struct ev_loop *\*(C'\fR) will not have
name \f(CW\*(C`loop\*(C'\fR (which is always of type \f(CW\*(C`ev_loop *\*(C'\fR) will not have
this argument.
.Sh "\s-1TIME\s0 \s-1REPRESENTATION\s0"
.IX Subsection "TIME REPRESENTATION"
@ -408,9 +408,13 @@ Example: This is basically the same thing that libev does internally, too.
.Ve
.SH "FUNCTIONS CONTROLLING THE EVENT LOOP"
.IX Header "FUNCTIONS CONTROLLING THE EVENT LOOP"
An event loop is described by a \f(CW\*(C`struct ev_loop *\*(C'\fR. The library knows two
types of such loops, the \fIdefault\fR loop, which supports signals and child
events, and dynamically created loops which do not.
An event loop is described by a \f(CW\*(C`struct ev_loop *\*(C'\fR (the \f(CW\*(C`struct\*(C'\fR
is \fInot\fR optional in this case, as there is also an \f(CW\*(C`ev_loop\*(C'\fR
\&\fIfunction\fR).
.PP
The library knows two types of such loops, the \fIdefault\fR loop, which
supports signals and child events, and dynamically created loops which do
not.
.IP "struct ev_loop *ev_default_loop (unsigned int flags)" 4
.IX Item "struct ev_loop *ev_default_loop (unsigned int flags)"
This will initialise the default event loop if it hasn't been initialised
@ -423,7 +427,7 @@ function.
.Sp
Note that this function is \fInot\fR thread-safe, so if you want to use it
from multiple threads, you have to lock (note also that this is unlikely,
as loops cannot bes hared easily between threads anyway).
as loops cannot be shared easily between threads anyway).
.Sp
The default loop is the only loop that can handle \f(CW\*(C`ev_signal\*(C'\fR and
\&\f(CW\*(C`ev_child\*(C'\fR watchers, and to do this, it always registers a handler
@ -508,26 +512,39 @@ This backend maps \f(CW\*(C`EV_READ\*(C'\fR to \f(CW\*(C`POLLIN | POLLERR | POLL
For few fds, this backend is a bit little slower than poll and select,
but it scales phenomenally better. While poll and select usually scale
like O(total_fds) where n is the total number of fds (or the highest fd),
epoll scales either O(1) or O(active_fds). The epoll design has a number
of shortcomings, such as silently dropping events in some hard-to-detect
cases and requiring a system call per fd change, no fork support and bad
support for dup.
epoll scales either O(1) or O(active_fds).
.Sp
The epoll mechanism deserves honorable mention as the most misdesigned
of the more advanced event mechanisms: mere annoyances include silently
dropping file descriptors, requiring a system call per change per file
descriptor (and unnecessary guessing of parameters), problems with dup and
so on. The biggest issue is fork races, however \- if a program forks then
\&\fIboth\fR parent and child process have to recreate the epoll set, which can
take considerable time (one syscall per file descriptor) and is of course
hard to detect.
.Sp
Epoll is also notoriously buggy \- embedding epoll fds \fIshould\fR work, but
of course \fIdoesn't\fR, and epoll just loves to report events for totally
\&\fIdifferent\fR file descriptors (even already closed ones, so one cannot
even remove them from the set) than registered in the set (especially
on \s-1SMP\s0 systems). Libev tries to counter these spurious notifications by
employing an additional generation counter and comparing that against the
events to filter out spurious ones, recreating the set when required.
.Sp
While stopping, setting and starting an I/O watcher in the same iteration
will result in some caching, there is still a system call per such incident
(because the fd could point to a different file description now), so its
best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed file descriptors might not work
very well if you register events for both fds.
.Sp
Please note that epoll sometimes generates spurious notifications, so you
need to use non-blocking I/O or other means to avoid blocking when no data
(or space) is available.
will result in some caching, there is still a system call per such
incident (because the same \fIfile descriptor\fR could point to a different
\&\fIfile description\fR now), so its best to avoid that. Also, \f(CW\*(C`dup ()\*(C'\fR'ed
file descriptors might not work very well if you register events for both
file descriptors.
.Sp
Best performance from this backend is achieved by not unregistering all
watchers for a file descriptor until it has been closed, if possible,
i.e. keep at least one watcher active per fd at all times. Stopping and
starting a watcher (without re-setting it) also usually doesn't cause
extra overhead.
extra overhead. A fork can both result in spurious notifications as well
as in libev having to destroy and recreate the epoll object, which can
take considerable time and thus should be avoided.
.Sp
While nominally embeddable in other event loops, this feature is broken in
all kernel versions tested so far.
@ -537,12 +554,15 @@ This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in th
.ie n .IP """EVBACKEND_KQUEUE"" (value 8, most \s-1BSD\s0 clones)" 4
.el .IP "\f(CWEVBACKEND_KQUEUE\fR (value 8, most \s-1BSD\s0 clones)" 4
.IX Item "EVBACKEND_KQUEUE (value 8, most BSD clones)"
Kqueue deserves special mention, as at the time of this writing, it was
broken on all BSDs except NetBSD (usually it doesn't work reliably with
anything but sockets and pipes, except on Darwin, where of course it's
completely useless). For this reason it's not being \*(L"auto-detected\*(R" unless
you explicitly specify it in the flags (i.e. using \f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or
libev was compiled on a known-to-be-good (\-enough) system like NetBSD.
Kqueue deserves special mention, as at the time of this writing, it
was broken on all BSDs except NetBSD (usually it doesn't work reliably
with anything but sockets and pipes, except on Darwin, where of course
it's completely useless). Unlike epoll, however, whose brokenness
is by design, these kqueue bugs can (and eventually will) be fixed
without \s-1API\s0 changes to existing programs. For this reason it's not being
\&\*(L"auto-detected\*(R" unless you explicitly specify it in the flags (i.e. using
\&\f(CW\*(C`EVBACKEND_KQUEUE\*(C'\fR) or libev was compiled on a known-to-be-good (\-enough)
system like NetBSD.
.Sp
You still can embed kqueue into a normal poll or select backend and use it
only for sockets (after having made sure that sockets work with kqueue on
@ -552,8 +572,9 @@ It scales in the same way as the epoll backend, but the interface to the
kernel is more efficient (which says nothing about its actual speed, of
course). While stopping, setting and starting an I/O watcher does never
cause an extra system call as with \f(CW\*(C`EVBACKEND_EPOLL\*(C'\fR, it still adds up to
two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad and it
drops fds silently in similarly hard-to-detect cases.
two event changes per incident. Support for \f(CW\*(C`fork ()\*(C'\fR is very bad (but
sane, unlike epoll) and it drops fds silently in similarly hard-to-detect
cases
.Sp
This backend usually performs well under most conditions.
.Sp
@ -592,7 +613,7 @@ might perform better.
On the positive side, with the exception of the spurious readiness
notifications, this backend actually performed fully to specification
in all tests and is fully embeddable, which is a rare feat among the
OS-specific backends.
OS-specific backends (I vastly prefer correctness over speed hacks).
.Sp
This backend maps \f(CW\*(C`EV_READ\*(C'\fR and \f(CW\*(C`EV_WRITE\*(C'\fR in the same way as
\&\f(CW\*(C`EVBACKEND_POLL\*(C'\fR.
@ -662,9 +683,9 @@ calling this function, or cope with the fact afterwards (which is usually
the easiest thing, you can just ignore the watchers and/or \f(CW\*(C`free ()\*(C'\fR them
for example).
.Sp
Note that certain global state, such as signal state, will not be freed by
this function, and related watchers (such as signal and child watchers)
would need to be stopped manually.
Note that certain global state, such as signal state (and installed signal
handlers), will not be freed by this function, and related watchers (such
as signal and child watchers) would need to be stopped manually.
.Sp
In general it is not advisable to call this function except in the
rare occasion where you really need to free e.g. the signal handling
@ -759,7 +780,7 @@ the loop.
A flags value of \f(CW\*(C`EVLOOP_ONESHOT\*(C'\fR will look for new events (waiting if
necessary) and will handle those and any already outstanding ones. It
will block your process until at least one new event arrives (which could
be an event internal to libev itself, so there is no guarentee that a
be an event internal to libev itself, so there is no guarantee that a
user-registered callback will be called), and will return after one
iteration of the loop.
.Sp
@ -843,7 +864,7 @@ Example: Create a signal watcher, but keep it from keeping \f(CW\*(C`ev_loop\*(C
running when nothing else is active.
.Sp
.Vb 4
\& struct ev_signal exitsig;
\& ev_signal exitsig;
\& ev_signal_init (&exitsig, sig_cb, SIGINT);
\& ev_signal_start (loop, &exitsig);
\& evf_unref (loop);
@ -904,7 +925,7 @@ they fire on, say, one-second boundaries only.
.IP "ev_loop_verify (loop)" 4
.IX Item "ev_loop_verify (loop)"
This function only does something when \f(CW\*(C`EV_VERIFY\*(C'\fR support has been
compiled in. which is the default for non-minimal builds. It tries to go
compiled in, which is the default for non-minimal builds. It tries to go
through all internal structures and checks them for validity. If anything
is found to be inconsistent, it will print an error message to standard
error and call \f(CW\*(C`abort ()\*(C'\fR.
@ -914,28 +935,38 @@ circumstances, this function will never abort as of course libev keeps its
data structures consistent.
.SH "ANATOMY OF A WATCHER"
.IX Header "ANATOMY OF A WATCHER"
In the following description, uppercase \f(CW\*(C`TYPE\*(C'\fR in names stands for the
watcher type, e.g. \f(CW\*(C`ev_TYPE_start\*(C'\fR can mean \f(CW\*(C`ev_timer_start\*(C'\fR for timer
watchers and \f(CW\*(C`ev_io_start\*(C'\fR for I/O watchers.
.PP
A watcher is a structure that you create and register to record your
interest in some event. For instance, if you want to wait for \s-1STDIN\s0 to
become readable, you would create an \f(CW\*(C`ev_io\*(C'\fR watcher for that:
.PP
.Vb 5
\& static void my_cb (struct ev_loop *loop, struct ev_io *w, int revents)
\& static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
\& {
\& ev_io_stop (w);
\& ev_unloop (loop, EVUNLOOP_ALL);
\& }
\&
\& struct ev_loop *loop = ev_default_loop (0);
\& struct ev_io stdin_watcher;
\&
\& ev_io stdin_watcher;
\&
\& ev_init (&stdin_watcher, my_cb);
\& ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
\& ev_io_start (loop, &stdin_watcher);
\&
\& ev_loop (loop, 0);
.Ve
.PP
As you can see, you are responsible for allocating the memory for your
watcher structures (and it is usually a bad idea to do this on the stack,
although this can sometimes be quite valid).
watcher structures (and it is \fIusually\fR a bad idea to do this on the
stack).
.PP
Each watcher has an associated watcher structure (called \f(CW\*(C`struct ev_TYPE\*(C'\fR
or simply \f(CW\*(C`ev_TYPE\*(C'\fR, as typedefs are provided for all watcher structs).
.PP
Each watcher structure must be initialised by a call to \f(CW\*(C`ev_init
(watcher *, callback)\*(C'\fR, which expects a callback to be provided. This
@ -943,19 +974,18 @@ callback gets invoked each time the event occurs (or, in the case of I/O
watchers, each time the event loop detects that the file descriptor given
is readable and/or writable).
.PP
Each watcher type has its own \f(CW\*(C`ev_<type>_set (watcher *, ...)\*(C'\fR macro
with arguments specific to this watcher type. There is also a macro
to combine initialisation and setting in one call: \f(CW\*(C`ev_<type>_init
(watcher *, callback, ...)\*(C'\fR.
Each watcher type further has its own \f(CW\*(C`ev_TYPE_set (watcher *, ...)\*(C'\fR
macro to configure it, with arguments specific to the watcher type. There
is also a macro to combine initialisation and setting in one call: \f(CW\*(C`ev_TYPE_init (watcher *, callback, ...)\*(C'\fR.
.PP
To make the watcher actually watch out for events, you have to start it
with a watcher-specific start function (\f(CW\*(C`ev_<type>_start (loop, watcher
with a watcher-specific start function (\f(CW\*(C`ev_TYPE_start (loop, watcher
*)\*(C'\fR), and you can stop watching for events at any time by calling the
corresponding stop function (\f(CW\*(C`ev_<type>_stop (loop, watcher *)\*(C'\fR.
corresponding stop function (\f(CW\*(C`ev_TYPE_stop (loop, watcher *)\*(C'\fR.
.PP
As long as your watcher is active (has been started but not stopped) you
must not touch the values stored in it. Most specifically you must never
reinitialise it or call its \f(CW\*(C`set\*(C'\fR macro.
reinitialise it or call its \f(CW\*(C`ev_TYPE_set\*(C'\fR macro.
.PP
Each and every callback receives the event loop pointer as first, the
registered watcher structure as second, and a bitset of received events as
@ -1032,8 +1062,12 @@ The given async watcher has been asynchronously notified (see \f(CW\*(C`ev_async
An unspecified error has occurred, the watcher has been stopped. This might
happen because the watcher could not be properly started because libev
ran out of memory, a file descriptor was found to be closed or any other
problem. You best act on it by reporting the problem and somehow coping
with the watcher being stopped.
problem. Libev considers these application bugs.
.Sp
You best act on it by reporting the problem and somehow coping with the
watcher being stopped. Note that well-written programs should not receive
an error ever, so when your watcher receives it, this usually indicates a
bug in your program.
.Sp
Libev will usually signal a few \*(L"dummy\*(R" events together with an error, for
example it might indicate that a fd is readable or writable, and if your
@ -1043,8 +1077,6 @@ programs, though, as the fd could already be closed and reused for another
thing, so beware.
.Sh "\s-1GENERIC\s0 \s-1WATCHER\s0 \s-1FUNCTIONS\s0"
.IX Subsection "GENERIC WATCHER FUNCTIONS"
In the following description, \f(CW\*(C`TYPE\*(C'\fR stands for the watcher type,
e.g. \f(CW\*(C`timer\*(C'\fR for \f(CW\*(C`ev_timer\*(C'\fR watchers and \f(CW\*(C`io\*(C'\fR for \f(CW\*(C`ev_io\*(C'\fR watchers.
.ie n .IP """ev_init"" (ev_TYPE *watcher, callback)" 4
.el .IP "\f(CWev_init\fR (ev_TYPE *watcher, callback)" 4
.IX Item "ev_init (ev_TYPE *watcher, callback)"
@ -1058,7 +1090,7 @@ which rolls both calls into one.
You can reinitialise a watcher at any time as long as it has been stopped
(or never started) and there are no pending events outstanding.
.Sp
The callback is always of type \f(CW\*(C`void (*)(ev_loop *loop, ev_TYPE *watcher,
The callback is always of type \f(CW\*(C`void (*)(struct ev_loop *loop, ev_TYPE *watcher,
int revents)\*(C'\fR.
.Sp
Example: Initialise an \f(CW\*(C`ev_io\*(C'\fR watcher in two steps.
@ -1164,7 +1196,7 @@ always \f(CW0\fR, which is supposed to not be too high and not be too low :).
.Sp
Setting a priority outside the range of \f(CW\*(C`EV_MINPRI\*(C'\fR to \f(CW\*(C`EV_MAXPRI\*(C'\fR is
fine, as long as you do not mind that the priority value you query might
or might not have been adjusted to be within valid range.
or might not have been clamped to the valid range.
.IP "ev_invoke (loop, ev_TYPE *watcher, int revents)" 4
.IX Item "ev_invoke (loop, ev_TYPE *watcher, int revents)"
Invoke the \f(CW\*(C`watcher\*(C'\fR with the given \f(CW\*(C`loop\*(C'\fR and \f(CW\*(C`revents\*(C'\fR. Neither
@ -1191,7 +1223,7 @@ data:
.Vb 7
\& struct my_io
\& {
\& struct ev_io io;
\& ev_io io;
\& int otherfd;
\& void *somedata;
\& struct whatever *mostinteresting;
@ -1206,7 +1238,7 @@ And since your callback will be called with a pointer to the watcher, you
can cast it back to your own type:
.PP
.Vb 5
\& static void my_cb (struct ev_loop *loop, struct ev_io *w_, int revents)
\& static void my_cb (struct ev_loop *loop, ev_io *w_, int revents)
\& {
\& struct my_io *w = (struct my_io *)w_;
\& ...
@ -1238,14 +1270,14 @@ programmers):
\& #include <stddef.h>
\&
\& static void
\& t1_cb (EV_P_ struct ev_timer *w, int revents)
\& t1_cb (EV_P_ ev_timer *w, int revents)
\& {
\& struct my_biggy big = (struct my_biggy *
\& (((char *)w) \- offsetof (struct my_biggy, t1));
\& }
\&
\& static void
\& t2_cb (EV_P_ struct ev_timer *w, int revents)
\& t2_cb (EV_P_ ev_timer *w, int revents)
\& {
\& struct my_biggy big = (struct my_biggy *
\& (((char *)w) \- offsetof (struct my_biggy, t2));
@ -1389,7 +1421,7 @@ attempt to read a whole line in the callback.
.PP
.Vb 6
\& static void
\& stdin_readable_cb (struct ev_loop *loop, struct ev_io *w, int revents)
\& stdin_readable_cb (struct ev_loop *loop, ev_io *w, int revents)
\& {
\& ev_io_stop (loop, w);
\& .. read from stdin here (or from w\->fd) and handle any I/O errors
@ -1397,7 +1429,7 @@ attempt to read a whole line in the callback.
\&
\& ...
\& struct ev_loop *loop = ev_default_init (0);
\& struct ev_io stdin_readable;
\& ev_io stdin_readable;
\& ev_io_init (&stdin_readable, stdin_readable_cb, STDIN_FILENO, EV_READ);
\& ev_io_start (loop, &stdin_readable);
\& ev_loop (loop, 0);
@ -1418,6 +1450,191 @@ The callback is guaranteed to be invoked only \fIafter\fR its timeout has
passed, but if multiple timers become ready during the same loop iteration
then order of execution is undefined.
.PP
\fIBe smart about timeouts\fR
.IX Subsection "Be smart about timeouts"
.PP
Many real-world problems involve some kind of timeout, usually for error
recovery. A typical example is an \s-1HTTP\s0 request \- if the other side hangs,
you want to raise some error after a while.
.PP
What follows are some ways to handle this problem, from obvious and
inefficient to smart and efficient.
.PP
In the following, a 60 second activity timeout is assumed \- a timeout that
gets reset to 60 seconds each time there is activity (e.g. each time some
data or other life sign was received).
.IP "1. Use a timer and stop, reinitialise and start it on activity." 4
.IX Item "1. Use a timer and stop, reinitialise and start it on activity."
This is the most obvious, but not the most simple way: In the beginning,
start the watcher:
.Sp
.Vb 2
\& ev_timer_init (timer, callback, 60., 0.);
\& ev_timer_start (loop, timer);
.Ve
.Sp
Then, each time there is some activity, \f(CW\*(C`ev_timer_stop\*(C'\fR it, initialise it
and start it again:
.Sp
.Vb 3
\& ev_timer_stop (loop, timer);
\& ev_timer_set (timer, 60., 0.);
\& ev_timer_start (loop, timer);
.Ve
.Sp
This is relatively simple to implement, but means that each time there is
some activity, libev will first have to remove the timer from its internal
data structure and then add it again. Libev tries to be fast, but it's
still not a constant-time operation.
.ie n .IP "2. Use a timer and re-start it with ""ev_timer_again"" inactivity." 4
.el .IP "2. Use a timer and re-start it with \f(CWev_timer_again\fR inactivity." 4
.IX Item "2. Use a timer and re-start it with ev_timer_again inactivity."
This is the easiest way, and involves using \f(CW\*(C`ev_timer_again\*(C'\fR instead of
\&\f(CW\*(C`ev_timer_start\*(C'\fR.
.Sp
To implement this, configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value
of \f(CW60\fR and then call \f(CW\*(C`ev_timer_again\*(C'\fR at start and each time you
successfully read or write some data. If you go into an idle state where
you do not expect data to travel on the socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR
the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will automatically restart it if need be.
.Sp
That means you can ignore both the \f(CW\*(C`ev_timer_start\*(C'\fR function and the
\&\f(CW\*(C`after\*(C'\fR argument to \f(CW\*(C`ev_timer_set\*(C'\fR, and only ever use the \f(CW\*(C`repeat\*(C'\fR
member and \f(CW\*(C`ev_timer_again\*(C'\fR.
.Sp
At start:
.Sp
.Vb 3
\& ev_timer_init (timer, callback);
\& timer\->repeat = 60.;
\& ev_timer_again (loop, timer);
.Ve
.Sp
Each time there is some activity:
.Sp
.Vb 1
\& ev_timer_again (loop, timer);
.Ve
.Sp
It is even possible to change the time-out on the fly, regardless of
whether the watcher is active or not:
.Sp
.Vb 2
\& timer\->repeat = 30.;
\& ev_timer_again (loop, timer);
.Ve
.Sp
This is slightly more efficient then stopping/starting the timer each time
you want to modify its timeout value, as libev does not have to completely
remove and re-insert the timer from/into its internal data structure.
.Sp
It is, however, even simpler than the \*(L"obvious\*(R" way to do it.
.IP "3. Let the timer time out, but then re-arm it as required." 4
.IX Item "3. Let the timer time out, but then re-arm it as required."
This method is more tricky, but usually most efficient: Most timeouts are
relatively long compared to the intervals between other activity \- in
our example, within 60 seconds, there are usually many I/O events with
associated activity resets.
.Sp
In this case, it would be more efficient to leave the \f(CW\*(C`ev_timer\*(C'\fR alone,
but remember the time of last activity, and check for a real timeout only
within the callback:
.Sp
.Vb 1
\& ev_tstamp last_activity; // time of last activity
\&
\& static void
\& callback (EV_P_ ev_timer *w, int revents)
\& {
\& ev_tstamp now = ev_now (EV_A);
\& ev_tstamp timeout = last_activity + 60.;
\&
\& // if last_activity + 60. is older than now, we did time out
\& if (timeout < now)
\& {
\& // timeout occured, take action
\& }
\& else
\& {
\& // callback was invoked, but there was some activity, re\-arm
\& // the watcher to fire in last_activity + 60, which is
\& // guaranteed to be in the future, so "again" is positive:
\& w\->again = timeout \- now;
\& ev_timer_again (EV_A_ w);
\& }
\& }
.Ve
.Sp
To summarise the callback: first calculate the real timeout (defined
as \*(L"60 seconds after the last activity\*(R"), then check if that time has
been reached, which means something \fIdid\fR, in fact, time out. Otherwise
the callback was invoked too early (\f(CW\*(C`timeout\*(C'\fR is in the future), so
re-schedule the timer to fire at that future time, to see if maybe we have
a timeout then.
.Sp
Note how \f(CW\*(C`ev_timer_again\*(C'\fR is used, taking advantage of the
\&\f(CW\*(C`ev_timer_again\*(C'\fR optimisation when the timer is already running.
.Sp
This scheme causes more callback invocations (about one every 60 seconds
minus half the average time between activity), but virtually no calls to
libev to change the timeout.
.Sp
To start the timer, simply initialise the watcher and set \f(CW\*(C`last_activity\*(C'\fR
to the current time (meaning we just have some activity :), then call the
callback, which will \*(L"do the right thing\*(R" and start the timer:
.Sp
.Vb 3
\& ev_timer_init (timer, callback);
\& last_activity = ev_now (loop);
\& callback (loop, timer, EV_TIMEOUT);
.Ve
.Sp
And when there is some activity, simply store the current time in
\&\f(CW\*(C`last_activity\*(C'\fR, no libev calls at all:
.Sp
.Vb 1
\& last_actiivty = ev_now (loop);
.Ve
.Sp
This technique is slightly more complex, but in most cases where the
time-out is unlikely to be triggered, much more efficient.
.Sp
Changing the timeout is trivial as well (if it isn't hard-coded in the
callback :) \- just change the timeout and invoke the callback, which will
fix things for you.
.IP "4. Wee, just use a double-linked list for your timeouts." 4
.IX Item "4. Wee, just use a double-linked list for your timeouts."
If there is not one request, but many thousands (millions...), all
employing some kind of timeout with the same timeout value, then one can
do even better:
.Sp
When starting the timeout, calculate the timeout value and put the timeout
at the \fIend\fR of the list.
.Sp
Then use an \f(CW\*(C`ev_timer\*(C'\fR to fire when the timeout at the \fIbeginning\fR of
the list is expected to fire (for example, using the technique #3).
.Sp
When there is some activity, remove the timer from the list, recalculate
the timeout, append it to the end of the list again, and make sure to
update the \f(CW\*(C`ev_timer\*(C'\fR if it was taken from the beginning of the list.
.Sp
This way, one can manage an unlimited number of timeouts in O(1) time for
starting, stopping and updating the timers, at the expense of a major
complication, and having to use a constant timeout. The constant timeout
ensures that the list stays sorted.
.PP
So which method the best?
.PP
Method #2 is a simple no-brain-required solution that is adequate in most
situations. Method #3 requires a bit more thinking, but handles many cases
better, and isn't very complicated either. In most case, choosing either
one is fine, with #3 being better in typical situations.
.PP
Method #1 is almost always a bad idea, and buys you nothing. Method #4 is
rather complicated, but extremely efficient, something that really pays
off after the first million or so of active timers, i.e. it's usually
overkill :)
.PP
\fIThe special problem of time updates\fR
.IX Subsection "The special problem of time updates"
.PP
@ -1472,38 +1689,8 @@ If the timer is started but non-repeating, stop it (as if it timed out).
If the timer is repeating, either start it if necessary (with the
\&\f(CW\*(C`repeat\*(C'\fR value), or reset the running timer to the \f(CW\*(C`repeat\*(C'\fR value.
.Sp
This sounds a bit complicated, but here is a useful and typical
example: Imagine you have a \s-1TCP\s0 connection and you want a so-called idle
timeout, that is, you want to be called when there have been, say, 60
seconds of inactivity on the socket. The easiest way to do this is to
configure an \f(CW\*(C`ev_timer\*(C'\fR with a \f(CW\*(C`repeat\*(C'\fR value of \f(CW60\fR and then call
\&\f(CW\*(C`ev_timer_again\*(C'\fR each time you successfully read or write some data. If
you go into an idle state where you do not expect data to travel on the
socket, you can \f(CW\*(C`ev_timer_stop\*(C'\fR the timer, and \f(CW\*(C`ev_timer_again\*(C'\fR will
automatically restart it if need be.
.Sp
That means you can ignore the \f(CW\*(C`after\*(C'\fR value and \f(CW\*(C`ev_timer_start\*(C'\fR
altogether and only ever use the \f(CW\*(C`repeat\*(C'\fR value and \f(CW\*(C`ev_timer_again\*(C'\fR:
.Sp
.Vb 8
\& ev_timer_init (timer, callback, 0., 5.);
\& ev_timer_again (loop, timer);
\& ...
\& timer\->again = 17.;
\& ev_timer_again (loop, timer);
\& ...
\& timer\->again = 10.;
\& ev_timer_again (loop, timer);
.Ve
.Sp
This is more slightly efficient then stopping/starting the timer each time
you want to modify its timeout value.
.Sp
Note, however, that it is often even more efficient to remember the
time of the last activity and let the timer time-out naturally. In the
callback, you then check whether the time-out is real, or, if there was
some activity, you reschedule the watcher to time-out in \*(L"last_activity +
timeout \- ev_now ()\*(R" seconds.
This sounds a bit complicated, see \*(L"Be smart about timeouts\*(R", above, for a
usage example.
.IP "ev_tstamp repeat [read\-write]" 4
.IX Item "ev_tstamp repeat [read-write]"
The current \f(CW\*(C`repeat\*(C'\fR value. Will be used each time the watcher times out
@ -1517,12 +1704,12 @@ Example: Create a timer that fires after 60 seconds.
.PP
.Vb 5
\& static void
\& one_minute_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
\& one_minute_cb (struct ev_loop *loop, ev_timer *w, int revents)
\& {
\& .. one minute over, w is actually stopped right here
\& }
\&
\& struct ev_timer mytimer;
\& ev_timer mytimer;
\& ev_timer_init (&mytimer, one_minute_cb, 60., 0.);
\& ev_timer_start (loop, &mytimer);
.Ve
@ -1532,12 +1719,12 @@ inactivity.
.PP
.Vb 5
\& static void
\& timeout_cb (struct ev_loop *loop, struct ev_timer *w, int revents)
\& timeout_cb (struct ev_loop *loop, ev_timer *w, int revents)
\& {
\& .. ten seconds without any activity
\& }
\&
\& struct ev_timer mytimer;
\& ev_timer mytimer;
\& ev_timer_init (&mytimer, timeout_cb, 0., 10.); /* note, only repeat used */
\& ev_timer_again (&mytimer); /* start timer */
\& ev_loop (loop, 0);
@ -1634,11 +1821,12 @@ If you need to stop it, return \f(CW\*(C`now + 1e30\*(C'\fR (or so, fudge fudge)
it afterwards (e.g. by starting an \f(CW\*(C`ev_prepare\*(C'\fR watcher, which is the
only event loop modification you are allowed to do).
.Sp
The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(struct ev_periodic
The callback prototype is \f(CW\*(C`ev_tstamp (*reschedule_cb)(ev_periodic
*w, ev_tstamp now)\*(C'\fR, e.g.:
.Sp
.Vb 4
\& static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now)
.Vb 5
\& static ev_tstamp
\& my_rescheduler (ev_periodic *w, ev_tstamp now)
\& {
\& return now + 60.;
\& }
@ -1682,8 +1870,8 @@ timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
The current interval value. Can be modified any time, but changes only
take effect when the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being
called.
.IP "ev_tstamp (*reschedule_cb)(struct ev_periodic *w, ev_tstamp now) [read\-write]" 4
.IX Item "ev_tstamp (*reschedule_cb)(struct ev_periodic *w, ev_tstamp now) [read-write]"
.IP "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read\-write]" 4
.IX Item "ev_tstamp (*reschedule_cb)(ev_periodic *w, ev_tstamp now) [read-write]"
The current reschedule callback, or \f(CW0\fR, if this functionality is
switched off. Can be changed any time, but changes only take effect when
the periodic timer fires or \f(CW\*(C`ev_periodic_again\*(C'\fR is being called.
@ -1697,12 +1885,12 @@ potentially a lot of jitter, but good long-term stability.
.PP
.Vb 5
\& static void
\& clock_cb (struct ev_loop *loop, struct ev_io *w, int revents)
\& clock_cb (struct ev_loop *loop, ev_io *w, int revents)
\& {
\& ... its now a full hour (UTC, or TAI or whatever your clock follows)
\& }
\&
\& struct ev_periodic hourly_tick;
\& ev_periodic hourly_tick;
\& ev_periodic_init (&hourly_tick, clock_cb, 0., 3600., 0);
\& ev_periodic_start (loop, &hourly_tick);
.Ve
@ -1713,7 +1901,7 @@ Example: The same as above, but use a reschedule callback to do it:
\& #include <math.h>
\&
\& static ev_tstamp
\& my_scheduler_cb (struct ev_periodic *w, ev_tstamp now)
\& my_scheduler_cb (ev_periodic *w, ev_tstamp now)
\& {
\& return now + (3600. \- fmod (now, 3600.));
\& }
@ -1724,7 +1912,7 @@ Example: The same as above, but use a reschedule callback to do it:
Example: Call a callback every hour, starting now:
.PP
.Vb 4
\& struct ev_periodic hourly_tick;
\& ev_periodic hourly_tick;
\& ev_periodic_init (&hourly_tick, clock_cb,
\& fmod (ev_now (loop), 3600.), 3600., 0);
\& ev_periodic_start (loop, &hourly_tick);
@ -1775,12 +1963,12 @@ Example: Try to exit cleanly on \s-1SIGINT\s0.
.PP
.Vb 5
\& static void
\& sigint_cb (struct ev_loop *loop, struct ev_signal *w, int revents)
\& sigint_cb (struct ev_loop *loop, ev_signal *w, int revents)
\& {
\& ev_unloop (loop, EVUNLOOP_ALL);
\& }
\&
\& struct ev_signal signal_watcher;
\& ev_signal signal_watcher;
\& ev_signal_init (&signal_watcher, sigint_cb, SIGINT);
\& ev_signal_start (loop, &signal_watcher);
.Ve
@ -1865,7 +2053,7 @@ its completion.
\& ev_child cw;
\&
\& static void
\& child_cb (EV_P_ struct ev_child *w, int revents)
\& child_cb (EV_P_ ev_child *w, int revents)
\& {
\& ev_child_stop (EV_A_ w);
\& printf ("process %d exited with status %x\en", w\->rpid, w\->rstatus);
@ -1890,8 +2078,9 @@ its completion.
.el .Sh "\f(CWev_stat\fP \- did the file attributes just change?"
.IX Subsection "ev_stat - did the file attributes just change?"
This watches a file system path for attribute changes. That is, it calls
\&\f(CW\*(C`stat\*(C'\fR regularly (or when the \s-1OS\s0 says it changed) and sees if it changed
compared to the last time, invoking the callback if it did.
\&\f(CW\*(C`stat\*(C'\fR on that path in regular intervals (or when the \s-1OS\s0 says it changed)
and sees if it changed compared to the last time, invoking the callback if
it did.
.PP
The path does not need to exist: changing from \*(L"path exists\*(R" to \*(L"path does
not exist\*(R" is a status change like any other. The condition \*(L"path does
@ -1899,17 +2088,18 @@ not exist\*(R" is signified by the \f(CW\*(C`st_nlink\*(C'\fR field being zero (
otherwise always forced to be at least one) and all the other fields of
the stat buffer having unspecified contents.
.PP
The path \fIshould\fR be absolute and \fImust not\fR end in a slash. If it is
relative and your working directory changes, the behaviour is undefined.
The path \fImust not\fR end in a slash or contain special components such as
\&\f(CW\*(C`.\*(C'\fR or \f(CW\*(C`..\*(C'\fR. The path \fIshould\fR be absolute: If it is relative and
your working directory changes, then the behaviour is undefined.
.PP
Since there is no standard kernel interface to do this, the portable
implementation simply calls \f(CW\*(C`stat (2)\*(C'\fR regularly on the path to see if
it changed somehow. You can specify a recommended polling interval for
this case. If you specify a polling interval of \f(CW0\fR (highly recommended!)
then a \fIsuitable, unspecified default\fR value will be used (which
you can expect to be around five seconds, although this might change
dynamically). Libev will also impose a minimum interval which is currently
around \f(CW0.1\fR, but thats usually overkill.
Since there is no portable change notification interface available, the
portable implementation simply calls \f(CWstat(2)\fR regularly on the path
to see if it changed somehow. You can specify a recommended polling
interval for this case. If you specify a polling interval of \f(CW0\fR (highly
recommended!) then a \fIsuitable, unspecified default\fR value will be used
(which you can expect to be around five seconds, although this might
change dynamically). Libev will also impose a minimum interval which is
currently around \f(CW0.1\fR, but that's usually overkill.
.PP
This watcher type is not meant for massive numbers of stat watchers,
as even with OS-supported change notifications, this can be
@ -1930,7 +2120,7 @@ structure. When using the library from programs that change the \s-1ABI\s0 to
use 64 bit file offsets the programs will fail. In that case you have to
compile libev with the same flags to get binary compatibility. This is
obviously the case with any flags that change the \s-1ABI\s0, but the problem is
most noticeably disabled with ev_stat and large file support.
most noticeably displayed with ev_stat and large file support.
.PP
The solution for this is to lobby your distribution maker to make large
file interfaces available by default (as e.g. FreeBSD does) and not
@ -1961,9 +2151,9 @@ etc. is difficult.
\fIThe special problem of stat time resolution\fR
.IX Subsection "The special problem of stat time resolution"
.PP
The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably, and
even on systems where the resolution is higher, most file systems still
only support whole seconds.
The \f(CW\*(C`stat ()\*(C'\fR system call only supports full-second resolution portably,
and even on systems where the resolution is higher, most file systems
still only support whole seconds.
.PP
That means that, if the time is the only thing that changes, you can
easily miss updates: on the first update, \f(CW\*(C`ev_stat\*(C'\fR detects a change and
@ -2125,14 +2315,14 @@ callback, free it. Also, use no error checking, as usual.
.PP
.Vb 7
\& static void
\& idle_cb (struct ev_loop *loop, struct ev_idle *w, int revents)
\& idle_cb (struct ev_loop *loop, ev_idle *w, int revents)
\& {
\& free (w);
\& // now do something you wanted to do when the program has
\& // no longer anything immediate to do.
\& }
\&
\& struct ev_idle *idle_watcher = malloc (sizeof (struct ev_idle));
\& ev_idle *idle_watcher = malloc (sizeof (ev_idle));
\& ev_idle_init (idle_watcher, idle_cb);
\& ev_idle_start (loop, idle_cb);
.Ve
@ -2223,13 +2413,13 @@ the callbacks for the IO/timeout watchers might not have been called yet.
\& static ev_timer tw;
\&
\& static void
\& io_cb (ev_loop *loop, ev_io *w, int revents)
\& io_cb (struct ev_loop *loop, ev_io *w, int revents)
\& {
\& }
\&
\& // create io watchers for each fd and a timer before blocking
\& static void
\& adns_prepare_cb (ev_loop *loop, ev_prepare *w, int revents)
\& adns_prepare_cb (struct ev_loop *loop, ev_prepare *w, int revents)
\& {
\& int timeout = 3600000;
\& struct pollfd fds [nfd];
@ -2254,7 +2444,7 @@ the callbacks for the IO/timeout watchers might not have been called yet.
\&
\& // stop all watchers after blocking
\& static void
\& adns_check_cb (ev_loop *loop, ev_check *w, int revents)
\& adns_check_cb (struct ev_loop *loop, ev_check *w, int revents)
\& {
\& ev_timer_stop (loop, &tw);
\&
@ -2435,7 +2625,7 @@ used).
.Vb 3
\& struct ev_loop *loop_hi = ev_default_init (0);
\& struct ev_loop *loop_lo = 0;
\& struct ev_embed embed;
\& ev_embed embed;
\&
\& // see if there is a chance of getting one that works
\& // (remember that a flags value of 0 means autodetection)
@ -2461,7 +2651,7 @@ kqueue implementation). Store the kqueue/socket\-only event loop in
.Vb 3
\& struct ev_loop *loop = ev_default_init (0);
\& struct ev_loop *loop_socket = 0;
\& struct ev_embed embed;
\& ev_embed embed;
\&
\& if (ev_supported_backends () & ~ev_recommended_backends () & EVBACKEND_KQUEUE)
\& if ((loop_socket = ev_loop_new (EVBACKEND_KQUEUE))
@ -2603,7 +2793,7 @@ employ a traditional mutex lock, such as in this pthread example:
.IP "ev_async_init (ev_async *, callback)" 4
.IX Item "ev_async_init (ev_async *, callback)"
Initialises and configures the async watcher \- it has no parameters of any
kind. There is a \f(CW\*(C`ev_asynd_set\*(C'\fR macro, but using it is utterly pointless,
kind. There is a \f(CW\*(C`ev_async_set\*(C'\fR macro, but using it is utterly pointless,
trust me.
.IP "ev_async_send (loop, ev_async *)" 4
.IX Item "ev_async_send (loop, ev_async *)"
@ -2668,17 +2858,17 @@ Example: wait up to ten seconds for data to appear on \s-1STDIN_FILENO\s0.
\&
\& ev_once (STDIN_FILENO, EV_READ, 10., stdin_ready, 0);
.Ve
.IP "ev_feed_event (ev_loop *, watcher *, int revents)" 4
.IX Item "ev_feed_event (ev_loop *, watcher *, int revents)"
.IP "ev_feed_event (struct ev_loop *, watcher *, int revents)" 4
.IX Item "ev_feed_event (struct ev_loop *, watcher *, int revents)"
Feeds the given event set into the event loop, as if the specified event
had happened for the specified watcher (which must be a pointer to an
initialised but not necessarily started event watcher).
.IP "ev_feed_fd_event (ev_loop *, int fd, int revents)" 4
.IX Item "ev_feed_fd_event (ev_loop *, int fd, int revents)"
.IP "ev_feed_fd_event (struct ev_loop *, int fd, int revents)" 4
.IX Item "ev_feed_fd_event (struct ev_loop *, int fd, int revents)"
Feed an event on the given fd, as if a file descriptor backend detected
the given events it.
.IP "ev_feed_signal_event (ev_loop *loop, int signum)" 4
.IX Item "ev_feed_signal_event (ev_loop *loop, int signum)"
.IP "ev_feed_signal_event (struct ev_loop *loop, int signum)" 4
.IX Item "ev_feed_signal_event (struct ev_loop *loop, int signum)"
Feed an event as if the given signal occurred (\f(CW\*(C`loop\*(C'\fR must be the default
loop!).
.SH "LIBEVENT EMULATION"
@ -2903,6 +3093,10 @@ more on top of it. It can be found via gem servers. Its homepage is at
.IX Item "D"
Leandro Lucarella has written a D language binding (\fIev.d\fR) for libev, to
be found at <http://proj.llucax.com.ar/wiki/evd>.
.IP "Ocaml" 4
.IX Item "Ocaml"
Erkki Seppala has written Ocaml bindings for libev, to be found at
<http://modeemi.cs.tut.fi/~flux/software/ocaml\-ev/>.
.SH "MACRO MAGIC"
.IX Header "MACRO MAGIC"
Libev can be compiled with a variety of options, the most fundamental
@ -3014,7 +3208,7 @@ where you can put other configuration options):
.Ve
.PP
Both header files and implementation files can be compiled with a \*(C+
compiler (at least, thats a stated goal, and breakage will be treated
compiler (at least, that's a stated goal, and breakage will be treated
as a bug).
.PP
You need the following files in your source tree, or in a directory
@ -3478,7 +3672,7 @@ you must not do this from \f(CW\*(C`ev_periodic\*(C'\fR reschedule callbacks.
.PP
Care has been taken to ensure that libev does not keep local state inside
\&\f(CW\*(C`ev_loop\*(C'\fR, and other calls do not usually allow for coroutine switches as
they do not clal any callbacks.
they do not call any callbacks.
.Sh "\s-1COMPILER\s0 \s-1WARNINGS\s0"
.IX Subsection "COMPILER WARNINGS"
Depending on your compiler and compiler settings, you might get no or a
@ -3521,7 +3715,7 @@ in libev, then check twice: If valgrind reports something like:
.Ve
.PP
Then there is no memory leak, just as memory accounted to global variables
is not a memleak \- the memory is still being refernced, and didn't leak.
is not a memleak \- the memory is still being referenced, and didn't leak.
.PP
Similarly, under some circumstances, valgrind might report kernel bugs
as if it were a bug in libev (e.g. in realloc or in the poll backend,
@ -3751,4 +3945,4 @@ calls in the current loop iteration. Checking for async and signal events
involves iterating over all running async watchers or all signal numbers.
.SH "AUTHOR"
.IX Header "AUTHOR"
Marc Lehmann <libev@schmorp.de>.
Marc Lehmann <libev@schmorp.de>, with repeated corrections by Mikael Magnusson.

45
ev.pod
View File

@ -388,27 +388,29 @@ but it scales phenomenally better. While poll and select usually scale
like O(total_fds) where n is the total number of fds (or the highest fd),
epoll scales either O(1) or O(active_fds).
The epoll syscalls are the most misdesigned of the more advanced event
mechanisms: problems include silently dropping fds, requiring a system
call per change per fd (and unnecessary guessing of parameters), problems
with dup and so on. The biggest issue is fork races, however - if a
program forks then I<both> parent and child process have to recreate the
epoll set, which can take considerable time (one syscall per fd) and is of
course hard to detect.
The epoll mechanism deserves honorable mention as the most misdesigned
of the more advanced event mechanisms: mere annoyances include silently
dropping file descriptors, requiring a system call per change per file
descriptor (and unnecessary guessing of parameters), problems with dup and
so on. The biggest issue is fork races, however - if a program forks then
I<both> parent and child process have to recreate the epoll set, which can
take considerable time (one syscall per file descriptor) and is of course
hard to detect.
Epoll is also notoriously buggy - embedding epoll fds should work, but
of course doesn't, and epoll just loves to report events for totally
Epoll is also notoriously buggy - embedding epoll fds I<should> work, but
of course I<doesn't>, and epoll just loves to report events for totally
I<different> file descriptors (even already closed ones, so one cannot
even remove them from the set) than registered in the set (especially
on SMP systems). Libev tries to counter these spurious notifications by
employing an additional generation counter and comparing that against the
events to filter out spurious ones.
events to filter out spurious ones, recreating the set when required.
While stopping, setting and starting an I/O watcher in the same iteration
will result in some caching, there is still a system call per such incident
(because the fd could point to a different file description now), so its
best to avoid that. Also, C<dup ()>'ed file descriptors might not work
very well if you register events for both fds.
will result in some caching, there is still a system call per such
incident (because the same I<file descriptor> could point to a different
I<file description> now), so its best to avoid that. Also, C<dup ()>'ed
file descriptors might not work very well if you register events for both
file descriptors.
Best performance from this backend is achieved by not unregistering all
watchers for a file descriptor until it has been closed, if possible,
@ -426,12 +428,15 @@ C<EVBACKEND_POLL>.
=item C<EVBACKEND_KQUEUE> (value 8, most BSD clones)
Kqueue deserves special mention, as at the time of this writing, it was
broken on all BSDs except NetBSD (usually it doesn't work reliably with
anything but sockets and pipes, except on Darwin, where of course it's
completely useless). For this reason it's not being "auto-detected" unless
you explicitly specify it in the flags (i.e. using C<EVBACKEND_KQUEUE>) or
libev was compiled on a known-to-be-good (-enough) system like NetBSD.
Kqueue deserves special mention, as at the time of this writing, it
was broken on all BSDs except NetBSD (usually it doesn't work reliably
with anything but sockets and pipes, except on Darwin, where of course
it's completely useless). Unlike epoll, however, whose brokenness
is by design, these kqueue bugs can (and eventually will) be fixed
without API changes to existing programs. For this reason it's not being
"auto-detected" unless you explicitly specify it in the flags (i.e. using
C<EVBACKEND_KQUEUE>) or libev was compiled on a known-to-be-good (-enough)
system like NetBSD.
You still can embed kqueue into a normal poll or select backend and use it
only for sockets (after having made sure that sockets work with kqueue on